St. Cyril on Dyoenergism and Dyotheletism

For things that have the same nature as one another will operate in the same way, while with things whose qualities have a different account the account of their operation in all respects would not be the same. (Commentary on the Holy Gospel According to John II.6, ed. Pusey, I, p. 318, 5-8)

Let the inquisitive again reflect that the Savior, in saying that His works bore clear witness that He was God by nature, taught plainly that it would not be among things possible for the operation and power fitting to God to exist in anything indistinguishably unless it too were God by nature. (ibid. III.1, ed. Pusey, I, p. 373, 10-15)

For it is, I think, clear and acknowledged by everyone that the properties of the Godhead are completely inaccessible to the created nature, and its natural attributes could never occur in any other existing thing in an equal and indistinguishable mode. (ibid. III.5, ed. Pusey, I, p. 448, 15-19)

‘If death can die without My dying’ (this clearly refers to the flesh) ‘let the cup depart’ (He says), ‘but’ (since it could not happen otherwise) ‘not as I will but as You will.’ You perceive how powerless again is human nature even in Christ, as it is found in itself; but is raised up to a courage proper to God the Word united to it. (ibid. IV.1, ed. Pusey, I, p. 487, 13-19)

If He perfected us through water and the Spirit, surely the same operated both divinely and humanly at the same time, being, in a single Being, both God and man together. (Commentary on Hebrews, lost fragment)

Things possessing the same operation and exercising the same natural powers must of necessity have wholly the same essence as well, for none of the things that exist will possess indistinguishably the same powers and operations as what is different in nature and different in essence. (Thesaurus, 8. PG 75. 105AB)

Things with the same operation are acknowledged to be of the same essence as well. (Thesaurus 10, PG 105. 137AB)

No sensible person would concede that things different in kind and nature possess the same operation. For fire could not have one and the same operation as water; but just as they possess a distinct definition of essence and qualities, so they will exhibit a different operation as well. (Thesaurus 14, PG 75. 241B)

When the Savior is shown dreading death and saying, ‘If it is possible, let this cup pass from me,’ (Mt. 26:39) reflect again that, when it was in dread of death, the flesh that was borne by God the Word was taught to suffer this no longer. He said to the Father, Not as I will, but as You will.’ (Mt. 26:39) For He did not fear death as Word and God, but was eager to perform the dispensation to the end, for such was the will of the Father. He had as well a volition not to die, because the flesh of its nature deprecated death. Therefore, teaching the manhood to think these thoughts no longer but to seek the will of God, He says as Man, ‘Not as I will but You will’. (Thesaurus 24, PG 75. 397A)

The Savior abolished death by His own death. For just as death would not have been abolished had He not died, so is it with each of the emotions of the flesh. For if He had not felt dread, nature would not be free of dread; if He had not felt distress, there would never have been an end to distress; if He had not been troubled and terrified, it would have never escaped from these things. Applying the same reasoning to each of the human experiences, you will find that the emotions of the flesh were excited in Christ not so that they might prevail as in us, but so that, once excited, they might be abolished by the power of the Word Who dwelt in the flesh, with nature being changed for the better. (Thesaurus 24, PG 75. 397C)

The Word of God became man not in order to perform and utter everything as God before the Incarnation, but so that often through the neediness of the dispensation with the flesh He might say certain things as man. Therefore, since the Mystery had this power, would it not be absurd for the hearers to take offense at His speaking, at times, in a more human way? Foe He speaks as man, and also speaks as God, having power to do both. (Thesaurus 24, PG 75. 400AB)

For how could One Who in works is equal to the Father be inferior as regards His nature? And how could One invested with the same operation and power as He is be different from Him in nature? (Thesaurus 32, PG 75. 453BC)